Banana sign

It is seen on axial imaging through the posterior fossa of fetus and is associated with the Chiari II malformation. It describes the way the cerebellum is wrapped tightly around the brain stem as a result of spinal cord tethering and downward migration of posterior fossa content. The cisterna magna gets obliterated and the shape of the cerebellum has the appearance of a banana. The sign persists into the second and third trimesters.

The banana sign is also seen in the majority of fetuses with spina bifida, but disappears after 24 weeks. There is very frequently concurrent hydrocephalus.